With this patch the configuration for distmem is simplified:

--disable-distmem (or --enable-distmem=no)
  disable distmem support entirely.
--enable-distmem=yes
  force enable distmem, autodetect back-ends, with an error if no back-end can be found.
--enable-distmem=auto (this is the defalt)
  enable distmem if and only if at least one back-end can be found.
--enable-distmem=x,y,z
  force enable distmem for back-ends x,y,z and fail if any cannot be found.

Additionally, when --enable-distmem=yes or --enable-distmem=auto is set, individual
back-ends can be ignored/disabled with --disable-distmem-xxx.